This invention relates generally to the field of metal working. More particularly, the invention relates to a method and apparatus for expanding or reducing a tubular workpiece by means of a high intensity magnetic field. The invention has a variety of applications including the manufacture of plate fin and tube heat exchangers.
Plate fin and tube type heat exchangers are widely used, particularly in engine cooling, air conditioning and refrigeration systems. Such a heat exchanger comprises a plurality of tubes through which a fluid, for example water or a refrigerant, flows. Another fluid, for example air, flows over the external surfaces of the tubes. Heat transfers between the internal and external fluids through the walls of the tubes. One factor that affects the rate of heat transfer between the fluids is the surface area of the tube that is in contact with the external fluid. One means commonly used to increase external surface area is to add plate fins to the tube. A plate fin is a strip or sheet of relatively thin material, usually metal, having holes through which the tubes of the heat exchanger pass. To be effective in improving the heat transfer performance of the heat exchanger, the plate fins must be in good physical contact with the tubes. During construction of a plate fin and tube heat exchanger, the holes in the plate fin are made with a diameter that is slightly larger than the outer diameter of the tubes. Then, after the fins are "laced" on to the tubes, the tubes are expanded radially to achieve a close mechanical fit with the fins.
There are a number of prior an methods of expanding tubes. One common method is mechanically, in which an expander tool is driven by some means, usually a rod, through the tube. The expander tool has an outer diameter that is larger than the inner diameter of the tube, so that driving the tool through enlarges the tube outer diameter sufficiently to make good contact with the fins. Frequently, the internal surface of a heat exchanger tube is not smooth but has some type of surface enhancement that serves to increase the internal surface area and promote fluid flow so as to improve the heat transfer performance of the tube. The expander tool used in mechanical expansion can damage the internal surface enhancement. During mechanical tube expansion, not only is the tube diameter increased but also the length of the tube decreases. The amount of the decrease varies so that after expansion, tubes that originally were all the same length can have different lengths. In making heat exchangers, this can present difficulties in subsequent manufacturing steps.
Advances in the heat exchanger art have led to the increasing use of relatively small diameter tubes. The diameter of the rod used to drive the expander tool in mechanical expansion apparatus must necessarily be smaller than the inner diameter of the tube being expanded. As tube diameters have decreased, the maximum allowable diameter of the expander rod has necessarily also decreased. As the expander rod diameter decreases, the likelihood of its buckling during an expansion operation increases. With the tube diameters now in consideration for use in heat exchangers, mechanical expansion using a rod-driven expander may not be possible. What is needed is a means for expanding very small diameter tubes.
In other manufacturing processes, there may be a requirement to reduce the diameter of a tube, such as joining a tube of one diameter to a tube of a smaller diameter.
U.S. Pat. No. 4,285,224, issued 25 Aug. 1981 to Shkatov et al., discloses an Electric Pulse Tube Expander. The Shkatov apparatus uses an "electric fuse" that explodes when an electric pulse is applied. The resultant shock wave acts through a filler in the fuse to cause the wall of the tube to expand. The Shkatov disclosure does not contain information regarding the aftermath of the explosion but it is probable that at least a certain amount of debris from the expansion process remains inside the tubes. Such debris must be removed from the tubes after expansion and before sealing the completed heat exchanger into a refrigeration or cooling system.
U.S. Pat. No. 4,947,667, issued 14 Aug. 1990 to Gunkel et al., for example discloses a Method and Apparatus for Reforming a Container. The Gunkel disclosure describes an apparatus and process for expanding selected portions of a cylindrical metal body using electromagnetic force. The Gunkel process requires a retainer. The Gunkel electrical circuit that generates the electromagnetic field does not include the workpiece.